1. Field of the Invention
The present invention relates generally to downhole tools, such as bridge and frac plugs, used to complete oil and/or gas wells.
2. Related Art
Oil and gas wells are completed using a complex process involving explosive charges and high pressure fluids. Once drilling is complete a well is lined with steel pipe backed with cement that bridges the gap between the pipe outer diameter (OD) and rock face. The steel/cement barrier is then perforated with explosive shaped charges. High pressure fluids and proppants (spherical sand or synthetic ceramic beads) are then pumped down the well, through the perforations and into the rock formation to prepare the rock for the flow of gas and oil into the casing and up the well. This fracturing process is repeated as many times as needed.
Another technological improvement has been the use of composite plugs used to complete these unconventional wells. Oil and gas wells are completed using a complex process whereby steel casing pipe is secured in place with cement. The steel/cement barrier and surrounding oil and gas bearing rock layers are then perforated with shaped charges in order to start the flow of oil and gas into the casing and up to the wellhead. As they prepare to perforate at each level, well technicians set a temporary plug in the bore of the steel casing pipe just below where they will perforate. This plug allows them to pump “Frac fluids” and sand down to the perforations and into the reservoir. This fractures the rock and props open the fractures allowing the movement of gas or oil towards the well at that level. Use of the temporary plug prevents contamination of already-fractured levels below. This process is repeated up the well until all desired zones have been stimulated. At each level, the temporary plugs are left in place, so that they can all be drilled out at the end of the process, in a single (but often time-consuming) operation. The ability to drill all the temporary composite plugs in a single pass (often taking only one day) compared to taking days or weeks to drill cast iron plugs has radically changed well completion economics.
Permanent and temporary plugs are locked to the casing using a system of cones and slips. The slip is typically made from cast iron or combinations of cast iron, ceramic buttons and composite materials. Each slip has hardened teeth or ceramic buttons that bite into the steel casing wall to lock the slip in place. The inside face usually consists of a conical surface that acts as a wedge. The slip's conical wedge face acts against a conical wedge formed by a cone. The cone is usually made from cast iron, aluminum or composite materials. The purpose of the cone is to act as a wedge to keep the slips locked in place and to provide support for the elastomeric elements used to seal the well bore.
Manufacturers use different designs to achieve this locking action and react the forces from the plug. Some manufacturers use a one piece cast iron slip and one piece cast iron cone. The slips have slots or grooves machined at equal intervals to assure the slips fracture when compressed and come in contact with the casing inner diameter (ID). The cones act as a conical wedge to fracture the slips and lock them in place against the casing wall. Such a cone-slip system does not assure equal spacing of the slip segments around the cone OD and casing ID. This causes uneven support of the cone and the plug to which it is connected. Examination of set plugs show gaps between slip segments can be as large as 1.5″ in a plug designed for 4.5″ casing. Further, as the surfaces of slip and cone contact each other they create extremely high point and line loads due to the contact profile created by unequal diameters of slip and cone. Cast iron plugs overcome these shortcomings with the brute force of massively over-designed cones and slips.
One manufacturer uses one piece cast iron slips and one piece composite cones made from fiberglass/epoxy material. The slips have slots or grooves which are used to set the breaking strength and spacing of the slip. The cones have brass pins used to crack and separate the broken slip segments. Such a cone-slip design can result in very high loads concentrated around a perimeter of contact between the cone and slip. At the beginning of the hydraulic fracturing process, the loads between cone and slip can be relatively light. As the temperature and pressure increases, the slip begins to crush and delaminate the cone as it presses itself into the cone (or deform the aluminum). Eventually, the cone can fail completely and the radial compressive loads from the slips transfer to the mandrel underneath the cone, whereupon, the mandrel begins to crush and fail.
Other manufacturers use a one piece cast iron slip with deep exterior grooves. These grooves allow the slip to fragment during the setting operation. The cone has a simple round conical outer diameter which acts against the conical slip to expand the slip segments and lock them to the casing wall. For example, see Magnum Oil Tools or Weatherford plugs. Such designs do not assure equal spacing of the slip segments around the cone and casing, causing uneven support of the cone and the plug to which it is connected. Further, as the surfaces of slip and cone contact each other they create extremely high point and line loads due to the contact profile created by unequal diameters of slip and cone.
Some manufacturers use a slip made of a cast iron toothed inserts molded to a composite backing piece. The slip segments are equally spaced around the plug circumference. For example, see Baker plugs. Such a design can assure that the slips are equally spaced around the cone to provide equal support to the cone and the plug body; but the composite material used as a support has a tendency to soften when exposed to the well fluids, high temperatures and pressures found in the well. The slip segments can be held together with non-metallic bands. For example, see BJ Services plugs. The slip segmented slips and backing rings can be held together with flat straps. When the plug is set the cables break and allow the slip segments to jump out to lock against the casing. For example, see Halliburton plugs. Such a design can assure that the slips are equally spaced around the cone to provide equal support to the cone and the plug body; but the flat straps can provide unreliable retention of the slip segments. If a strap loosens or breaks then the slip segment can catch against the casing wall and cause a premature set. A premature set causes the tool string (i.e. perforating guns, setting tool and plug) to become stuck. A stuck tool string costs tens to hundreds of thousands of dollars in direct and opportunity costs to remove. Some cone-slip system consists of a layer of segmented cast iron pieces with aluminum supports held together with a metal ring installed on the inside radius. The segments separate when the plug is set and move outward until they touch the casing. For example, see Smith Services plugs. Such a cone-slip system is entirely made from metal, which are often rejected by operators for their real or perceived long drill out times. Such a design can also have numerous and/or complex pieces used in the cone-slip system. All the designs with slip segments acting against a cone having flat facets also have flat facets separated by ridges equally spaced about the circumference of the cone.
When the plug is set, a setting sleeve compresses the stack of slips, cones and rubber elements. The rubber elements expand outward and inward and create a seal between the elements and mandrel and the elements and the inner diameter of the well casing. The rubber elements also act on one to two layers of sheet metal petals and force them into contact with the inner diameter of the steel casing. This prevents the rubber elements from extruding past the petals. The lock ring engages the threads in the mandrel and the threads in the push sleeve to prevent backward (i.e. upward) movement once the force from the setting tool is released. This locking action keeps pressure on the elements which preserves the seal and keeps the slips locked to the ID of the casing. This blocks fluid from getting to the lower layers of rock and creates the seal needed to perform hydraulic fracturing in the layers above the plug.
Examples of downhole tools include US Patent Publication No. 2011/0079383; and U.S. Pat. Nos. 4,926,938; 5,540,279; 6,491,108; and 6,695,050.